Senior Backend Engineer - (Kotlin/JVM)

SumUp builds simple, affordable financial tools that help over 4 million small businesses across 37 markets get paid, manage their money, and grow. Beyond card payments, we offer a full suite of financial products - business banking, lending, invoicing, and money transfers - giving merchants everything they need in one place. We're a global fintech with over 3,000 people across 20+ offices, and Sofia has been one of our most important engineering hubs since the very beginning. Our Transfers Excellence squad sits within SumUp's Global Bank , and owns the core services that power how merchants and consumers move money. E.g. IBAN-based transfers, money in and out, and peer-to-peer payments via the SumUp consumer app. With the high volume transfers per hour across our European markets. That means reliability isn't a nice-to-have - it's the baseline for us. Right now, the team is replacing a set of market-specific implementations with a single, unified global transfer platform that will serve Europe, the UK, Brazil, Chile, and eventually the US and Canada. Join our team and contribute directly to platform-level architecture that will shape how SumUp moves money at scale - work that directly impacts millions of small businesses every day. Our tech stack includes Kotlin, Java, Go, a bit of Elixir, AWS, Kafka, PostgreSQL, and Kubernetes, supported by a strong observability toolchain with Prometheus, Grafana, and Honeycomb. We also actively use AI‑assisted development tools such as Cursor, GitHub Copilot, and others. 🎥 Get a feel for life in our Sofia office . What you'll do Build and maintain backend services that sit at the heart of SumUp's transfer infrastructure, taking end-to-end ownership from development through to observability, testing and production support Contribute to the migration from market-specific services toward a unified global transfer platform, working across a mix of active services and legacy systems being decomposed Raise well-scoped pull requests, conduct thorough code reviews, and help maintain high standards of code quality and test coverage across the team Participate in incident reviews and the on-call rotation, taking accountability for the reliability of the services you build Contribute to the technical roadmap by participating in architectural discussions and proposing solutions for complex, distributed systems challenges Apply AI tooling to accelerate your own engineering workflows and contribute to raising the team's overall AI literacy You'll be great for this role if… Solid experience as a Backend Software Engineer in modern languages (e.g. Kotlin, Go, Java, etc.). Kotlin is the team's primary language and we also work with Java, Golang, and Elixir. Solid understanding of distributed systems and event-driven architecture. Experience with async processing and messaging tools like Kafka (or similar). Experience working with microservice architectures, including Spring Boot and event-driven patterns, alongside solid knowledge of relational databases and API design Comfortable working in high-throughput, regulated environments where reliability matters. Defensive coding and attention to edge cases are part of the job Clear communicator in written and spoken English, with a collaborative approach to working across global teams Curious about the product impact, not just the code. Comfortable thinking about how backend decisions affect the end user experience. Why you should join SumUp 🌍 Opportunity to work with SumUppers globally on large-scale fintech products used by millions of businesses worldwide, from our modern Sofia office. This involves an office-first setup 🌈 Commitment to Diversity and Inclusion: be part of a workplace that values and promotes diversity, fostering an inclusive environment where everyone's perspectives are respected and embraced 🚀 Enrolment onto our Virtual Stock Option programme: you will own a stake in SumUp's future success 📚 A dedicated annual L&D budget of €2000 and...